Method and apparatus for transmitting a reference signal in a multi-antenna system

ABSTRACT

Provided are a method and apparatus for transmitting a reference signal in a multi-antenna system. A terminal generates a plurality of reference signal sequences in which cyclic shift values different from each other are allocated to the respective plurality of layers, and generates a single carrier-frequency division multiple access (SC-FDMA) symbol to which the plurality of reference signal sequences are mapped. The SC-FDMA symbol is transmitted to a base station via a plurality of antennas. The cyclic shift values allocated to the respective layers are determined on the basis of a first cyclic shift value, which is a cyclic shift value allocated to a first layer from among the plurality of layers, and cyclic shift offsets which are allocated to the respective layers and which are different from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting a referencesignal in a multi-antenna system.

2. Related Art

Effective transmission/reception methods and utilizations have beenproposed for a broadband wireless communication system to maximizeefficiency of radio resources. An orthogonal frequency divisionmultiplexing (OFDM) system capable of reducing inter-symbol interference(ISI) with a low complexity is taken into consideration as one of nextgeneration wireless communication systems. In the OFDM, a serially inputdata symbol is converted into N parallel data symbols, and is thentransmitted by being carried on each of separated N subcarriers. Thesubcarriers maintain orthogonality in a frequency dimension. Eachorthogonal channel experiences mutually independent frequency selectivefading, and an interval of a transmitted symbol is increased, therebyminimizing inter-symbol interference.

When a system uses the OFDM as a modulation scheme, orthogonal frequencydivision multiple access (OFDMA) is a multiple access scheme in whichmultiple access is achieved by independently providing some of availablesubcarriers to a plurality of users. In the OFDMA, frequency resources(i.e., subcarriers) are provided to the respective users, and therespective frequency resources do not overlap with one another ingeneral since they are independently provided to the plurality of users.Consequently, the frequency resources are allocated to the respectiveusers in a mutually exclusive manner. In an OFDMA system, frequencydiversity for multiple users can be obtained by using frequencyselective scheduling, and subcarriers can be allocated variouslyaccording to a permutation rule for the subcarriers. In addition, aspatial multiplexing scheme using multiple antennas can be used toincrease efficiency of a spatial domain.

MIMO technology can be used to improve the efficiency of datatransmission and reception using multiple transmission antennas andmultiple reception antennas. MIMO technology may include a spacefrequency block code (SFBC), a space time block code (STBC), a cyclicdelay diversity (CDD), a frequency switched transmit diversity (FSTD), atime switched transmit diversity (TSTD), a precoding vector switching(PVS), spatial multiplexing (SM) for implementing diversity. An MIMOchannel matrix according to the number of reception antennas and thenumber of transmission antennas can be decomposed into a number ofindependent channels. Each of the independent channels is called a layeror stream. The number of layers is called a rank.

In wireless communication systems, it is necessary to estimate an uplinkchannel or a downlink channel for the purpose of the transmission andreception of data, the acquisition of system synchronization, and thefeedback of channel information. In wireless communication systemenvironments, fading is generated because of multi-path time latency. Aprocess of restoring a transmit signal by compensating for thedistortion of the signal resulting from a sudden change in theenvironment due to such fading is referred to as channel estimation. Itis also necessary to measure the state of a channel for a cell to whicha user equipment belongs or other cells. To estimate a channel ormeasure the state of a channel, a reference signal (RS) which is knownto both a transmitter and a receiver can be used.

A subcarrier used to transmit the reference signal is referred to as areference signal subcarrier, and a subcarrier used to transmit data isreferred to as a data subcarrier. In an OFDM system, a method ofassigning the reference signal includes a method of assigning thereference signal to all the subcarriers and a method of assigning thereference signal between data subcarriers. The method of assigning thereference signal to all the subcarriers is performed using a signalincluding only the reference signal, such as a preamble signal, in orderto obtain the throughput of channel estimation. If this method is used,the performance of channel estimation can be improved as compared withthe method of assigning the reference signal between data subcarriersbecause the density of reference signals is in general high. However,since the amount of transmitted data is small in the method of assigningthe reference signal to all the subcarriers, the method of assigning thereference signal between data subcarriers is used in order to increasethe amount of transmitted data. If the method of assigning the referencesignal between data subcarriers is used, the performance of channelestimation can be deteriorated because the density of reference signalsis low. Accordingly, the reference signals should be properly arrangedin order to minimize such deterioration.

A receiver can estimate a channel by separating information about areference signal from a received signal because it knows the informationabout a reference signal and can accurately estimate data, transmittedby a transmit stage, by compensating for an estimated channel value.Assuming that the reference signal transmitted by the transmitter is p,channel information experienced by the reference signal duringtransmission is h, thermal noise occurring in the receiver is n, and thesignal received by the receiver is y, it can result in y=h·p+n. Here,since the receiver already knows the reference signal p, it can estimatea channel information value ĥ using Equation 1 in the case in which aleast square (LS) method is used.

ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]

The accuracy of the channel estimation value ĥ estimated using thereference signal p is determined by the value {circumflex over (n)}. Toaccurately estimate the value h, the value {circumflex over (n)} mustconverge on 0. To this end, the influence of the value {circumflex over(n)} has to be minimized by estimating a channel using a large number ofreference signals. A variety of algorithms for a better channelestimation performance may exist.

Meanwhile, a reference signal transmission method supporting a MIMOsystem using a plurality of antennas in uplink transmission and itsrelated method of allocating a cyclic shift value of a reference signalsequence have not been proposed up to now in the 3GPP LTE system.Therefore, there is a need for a reference signal transmission methodwhich guarantees channel estimation capability in the MIMO system.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for transmitting areference signal in a multi-antenna system.

In an aspect, a method of transmitting a reference signal in amulti-antenna system is provided. The method includes generating aplurality of reference signal sequences in which different cyclic shiftvalues are allocated respectively to a plurality of layers, generating asingle carrier-frequency division multiple access (SC-FDMA) symbol towhich the plurality of reference signal sequences are mapped, andtransmitting the SC-FDMA symbol to a base station through a plurality ofantennas, wherein the cyclic shift values allocated to the respectivelayers are determined by a first cyclic shift value which is a cyclicshift value allocated to a first layer among the plurality of layers anddifferent cyclic shift offsets allocated to the respective layers. Thefirst cyclic shift value and a second cyclic shift value which is acyclic shift value allocated to a second layer among the plurality oflayers may have a maximum interval. The number of the plurality oflayers may be 3. A third cyclic shift offset which is a cyclic shiftoffset allocated to a third layer among the plurality of layers may be amedian value of the first cyclic shift offset which is the cyclic shiftoffset allocated to the first layer among the plurality of layers andthe second cyclic shift offset which is the cyclic shift offsetallocated to the second layer. The first cyclic shift offset, the secondcyclic shift offset, and the third cyclic shift offset may berespectively 0, 6, and 3. If the number of the plurality of layers is 4,a third cyclic shift value which is a cyclic shift value allocated to athird layer among the plurality of layers and a fourth cyclic shiftvalue which is a cyclic shift value allocated to a fourth layer may havea maximum interval. The cyclic shift values of the reference signals forthe plurality of layers may be indicated by a cyclic shift field in adownlink control information (DCI) format transmitted through a physicaldownlink control channel (PDCCH). The cyclic shift field may have alength of 3 bits. The plurality of reference signal sequences may betransmitted in two slots constituting a subframe. The plurality ofreference signal sequences may be transmitted in a fourth SC-FDMA symbolof each slot in case of a normal cyclic prefix (CP), and the pluralityof reference signal sequences may be transmitted in a third SC-FDMAsymbol of each slot in case of an extended CP. An orthogonal coveringcode (OCC) may be applied to the reference signal sequences for theplurality of layers.

In another aspect, an apparatus for transmitting a reference signal isprovided. The apparatus includes a reference signal generator configuredfor generating a plurality of reference signal sequences in whichdifferent cyclic shift values are allocated respectively to a pluralityof layers, an SC-FDMA symbol generator configured for generating anSC-FDMA symbol to which the plurality of reference signal sequences aremapped, and a radio frequency (RF) unit configured for transmitting theSC-FDMA symbol to a base station through a plurality of antennas,wherein the cyclic shift values allocated to the respective layers aredetermined by a first cyclic shift value which is a cyclic shift valueallocated to a first layer among the plurality of layers and differentcyclic shift offsets allocated to the respective layers. The firstcyclic shift value and a second cyclic shift value which is a cyclicshift value allocated to a second layer among the plurality of layersmay have a maximum interval. A third cyclic shift offset which is acyclic shift offset allocated to a third layer among the plurality oflayers may be a median value of the first cyclic shift offset which isthe cyclic shift offset allocated to the first layer among the pluralityof layers and the second cyclic shift offset which is the cyclic shiftoffset allocated to the second layer. The first cyclic shift offset, thesecond cyclic shift offset, and the third cyclic shift offset may berespectively 0, 6, and 3.

By effectively allocating a cyclic shift value for a reference signalsequence, channel estimation capability can be guaranteed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

FIG. 3 shows an example of a resource grid of a single downlink slot.

FIG. 4 shows the structure of a downlink subframe.

FIG. 5 shows the structure of an uplink subframe.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMAsystem.

FIG. 7 shows an example of a scheme in which the subcarrier mapper mapsthe complex-valued symbols to the respective subcarriers of thefrequency domain.

FIG. 8 shows an example of the structure of a reference signaltransmitter for demodulation.

FIG. 9 shows examples of a subframe through which a reference signal istransmitted.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDMtransmission scheme.

FIG. 11 shows another example of a transmitter using the clustered DFT-sOFDM transmission scheme.

FIG. 12 is yet another example of a transmitter using the clusteredDFT-s OFDM transmission scheme.

FIG. 13 is a block diagram showing an embodiment of the proposedreference signal transmission method.

FIG. 14 is a block diagram of a UE according to an embodiment of thepresent invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following technique may be used for various wireless communicationsystems such as code division multiple access (CDMA), a frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), singlecarrier-frequency division multiple access (SC-FDMA), and the like. TheCDMA may be implemented as a radio technology such as universalterrestrial radio access (UTRA) or CDMA2000. The TDMA may be implementedas a radio technology such as a global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by a radio technologysuch as IEEE (Institute of Electrical and Electronics Engineers) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (Evolved UTRA), andthe like. IEEE 802.16m, an evolution of IEEE 802.16e, provides backwardcompatibility with a system based on IEEE 802.16e. The UTRA is part of auniversal mobile telecommunications system (UMTS). 3GPP (3rd generationpartnership project) LTE (long term evolution) is part of an evolvedUMTS (E-UMTS) using the E-UTRA, which employs the OFDMA in downlink andthe SC-FDMA in uplink. LTE-A (advanced) is an evolution of 3GPP LTE.

Hereinafter, for clarification, LET-A will be largely described, but thetechnical concept of the present invention is not meant to be limitedthereto.

FIG. 1 shows a wireless communication system.

The wireless communication system 10 includes at least one base station(BS) 11. Respective BSs 11 provide a communication service to particulargeographical areas 15 a, 15 b, and 15 c (which are generally calledcells). Each cell may be divided into a plurality of areas (which arecalled sectors). A user equipment (UE) 12 may be fixed or mobile and maybe referred to by other names such as MS (mobile station), MT (mobileterminal), UT (user terminal), SS (subscriber station), wireless device,PDA (personal digital assistant), wireless modem, handheld device. TheBS 11 generally refers to a fixed station that communicates with the UE12 and may be called by other names such as eNB (evolved-NodeB), BTS(base transceiver system), access point (AP), etc.

In general, a UE belongs to one cell, and the cell to which a UE belongsis called a serving cell. A BS providing a communication service to theserving cell is called a serving BS. The wireless communication systemis a cellular system, so a different cell adjacent to the serving cellexists. The different cell adjacent to the serving cell is called aneighbor cell. A BS providing a communication service to the neighborcell is called a neighbor BS. The serving cell and the neighbor cell arerelatively determined based on a UE.

This technique can be used for downlink or uplink. In general, downlinkrefers to communication from the BS 11 to the UE 12, and uplink refersto communication from the UE 12 to the BS 11. In downlink, a transmittermay be part of the BS 11 and a receiver may be part of the UE 12. Inuplink, a transmitter may be part of the UE 12 and a receiver may bepart of the BS 11.

The wireless communication system may be any one of a multiple-inputmultiple-output (MIMO) system, a multiple-input single-output (MISO)system, a single-input single-output (SISO) system, and a single-inputmultiple-output (SIMO) system. The MIMO system uses a plurality oftransmission antennas and a plurality of reception antennas. The MISOsystem uses a plurality of transmission antennas and a single receptionantenna. The SISO system uses a single transmission antenna and a singlereception antenna. The SIMO system uses a single transmission antennaand a plurality of reception antennas.

Hereinafter, a transmission antenna refers to a physical or logicalantenna used for transmitting a signal or a stream, and a receptionantenna refers to a physical or logical antenna used for receiving asignal or a stream.

FIG. 2 shows the structure of a radio frame in 3GPP LTE. It may bereferred to Paragraph 5 of “Technical Specification Group Radio AccessNetwork; Evolved Universal Terrestrial Radio Access (E-UTRA); Physicalchannels and modulation (Release 8)” to 3GPP (3rd generation partnershipproject) TS 36.211 V8.2.0 (2008-03).

Referring to FIG. 2, the radio frame includes 10 subframes, and onesubframe includes two slots. The slots in the radio frame are numberedby #0 to #19. A time taken for transmitting one subframe is called atransmission time interval (TTI). The TTI may be a scheduling unit for adata transmission. For example, a radio frame may have a length of 10ms, a subframe may have a length of 1 ms, and a slot may have a lengthof 0.5 ms.

One slot includes a plurality of OFDM (Orthogonal Frequency DivisionMultiplexing) symbols in a time domain and a plurality of subcarriers ina frequency domain. Since 3GPP LTE uses OFDMA in downlink, the OFDMsymbols are used to express a symbol period. The OFDM symbols may becalled by other names depending on a multiple-access scheme. Forexample, when SC-FDMA is in use as an uplink multi-access scheme, theOFDM symbols may be called SC-FDMA symbols. A resource block (RB), aresource allocation unit, includes a plurality of continuous subcarriersin a slot. The structure of the radio frame is merely an example.Namely, the number of subframes included in a radio frame, the number ofslots included in a subframe, or the number of OFDM symbols included ina slot may vary.

3GPP LTE defines that one slot includes seven OFDM symbols in a normalcyclic prefix (CP) and one slot includes six OFDM symbols in an extendedCP.

FIG. 3 shows an example of a resource grid of a single downlink slot.

A downlink slot includes a plurality of OFDM symbols in the time domainand NRB number of resource blocks (RBs) in the frequency domain. The NRBnumber of resource blocks included in the downlink slot is dependentupon a downlink transmission bandwidth set in a cell. For example, in anLTE system, NRB may be any one of 60 to 110. One resource block includesa plurality of subcarriers in the frequency domain. An uplink slot mayhave the same structure as that of the downlink slot.

Each element on the resource grid is called a resource element. Theresource elements on the resource grid can be discriminated by a pair ofindexes (k,l) in the slot. Here, k (k=0, . . . , NRB×12−1) is asubcarrier index in the frequency domain, and 1 is an OFDM symbol indexin the time domain.

Here, it is illustrated that one resource block includes 7×12 resourceelements made up of seven OFDM symbols in the time domain and twelvesubcarriers in the frequency domain, but the number of OFDM symbols andthe number of subcarriers in the resource block are not limited thereto.The number of OFDM symbols and the number of subcarriers may varydepending on the length of a cyclic prefix (CP), frequency spacing, andthe like. For example, in case of a normal CP, the number of OFDMsymbols is 7, and in case of an extended CP, the number of OFDM symbolsis 6. One of 128, 256, 512, 1024, 1536, and 2048 may be selectively usedas the number of subcarriers in one OFDM symbol.

FIG. 4 shows the structure of a downlink subframe.

A downlink subframe includes two slots in the time domain, and each ofthe slots includes seven OFDM symbols in the normal CP. First three OFDMsymbols (maximum four OFDM symbols with respect to a 1.4 Mhz bandwidth)of a first slot in the subframe corresponds to a control region to whichcontrol channels are allocated, and the other remaining OFDM symbolscorrespond to a data region to which a physical downlink shared channel(PDSCH) is allocated.

The PDCCH may carry a transmission format and a resource allocation of adownlink shared channel (DL-SCH), resource allocation information of anuplink shared channel (UL-SCH), paging information on a PCH, systeminformation on a DL-SCH, a resource allocation of an higher layercontrol message such as a random access response transmitted via aPDSCH, a set of transmission power control commands with respect toindividual UEs in a certain UE group, an activation of a voice overinternet protocol (VoIP), and the like. A plurality of PDCCHs may betransmitted in the control region, and a UE can monitor a plurality ofPDCCHs. The PDCCHs are transmitted on one or an aggregation of aplurality of consecutive control channel elements (CCE). The CCE is alogical allocation unit used to provide a coding rate according to thestate of a wireless channel. The CCD corresponds to a plurality ofresource element groups. The format of the PDCCH and an available numberof bits of the PDCCH are determined according to an associative relationbetween the number of the CCEs and a coding rate provided by the CCEs.

The BS determines a PDCCH format according to a DCI to be transmitted tothe UE, and attaches a cyclic redundancy check (CRC) to the DCI. Aunique radio network temporary identifier (RNTI) is masked on the CRCaccording to the owner or the purpose of the PDCCH. In case of a PDCCHfor a particular UE, a unique identifier, e.g., a cell-RNTI (C-RNTI), ofthe UE, may be masked on the CRC. Or, in case of a PDCCH for a pagingmessage, a paging indication identifier, e.g., a paging-RNTI (P-RNTI),may be masked on the CRC. In case of a PDCCH for a system informationblock (SIB), a system information identifier, e.g., a systeminformation-RNTI (SI-RNTI), may be masked on the CRC. In order toindicate a random access response, i.e., a response to a transmission ofa random access preamble of the UE, a random access-RNTI (RA-RNTI) maybe masked on the CRC.

FIG. 5 shows the structure of an uplink subframe.

An uplink subframe may be divided into a control region and a dataregion in the frequency domain. A physical uplink control channel(PUCCH) for transmitting uplink control information is allocated to thecontrol region. A physical uplink shared channel (PUCCH) fortransmitting data is allocated to the data region. The user equipmentdoes not transmit the PUCCH and the PUSCH simultaneously to maintain asingle carrier property.

The PUCCH with respect to a UE is allocated by a pair of resource blocksin a subframe. The resource blocks belonging to the pair of resourceblocks (RBs) occupy different subcarriers in first and second slots,respectively. The frequency occupied by the RBs belonging to the pair ofRBs is changed based on a slot boundary. This is said that the pair ofRBs allocated to the PUCCH are frequency-hopped at the slot boundary.The UE can obtain a frequency diversity gain by transmitting uplinkcontrol information through different subcarriers according to time. InFIG. 5, m is a position index indicating the logical frequency domainpositions of the pair of RBs allocated to the PUCCH in the subframe.

Uplink control information transmitted on the PUCCH may include a hybridautomatic repeat request (HARQ) acknowledgement/non-acknowledgement(ACK/NACK), a channel quality indicator (CQI) indicating the state of adownlink channel, an scheduling request (SR), and the like.

The PUSCH is mapped to a uplink shared channel (UL-SCH), a transportchannel. Uplink data transmitted on the PUSCH may be a transport block,a data block for the UL-SCH transmitted during the TTI. The transportblock may be user information. Or, the uplink data may be multiplexeddata. The multiplexed data may be data obtained by multiplexing thetransport block for the UL-SCH and control information. For example,control information multiplexed to data may include a CQI, a precodingmatrix indicator (PMI), an HARQ, a rank indicator (RI), or the like. Orthe uplink data may include only control information.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMAsystem.

Referring to FIG. 6, the transmitter 50 includes a discrete Fouriertransform (DFT) unit 51, a subcarrier mapper 52, an inverse fast Fouriertransform (IFFT) unit 53, and a cyclic prefix (CP) insertion unit 54.The transmitter 50 may include a scramble unit (not shown), a modulationmapper (not shown), a layer mapper (not shown), and a layer permutator(not shown), which may be placed in front of the DFT unit 51.

The DFT unit 51 outputs complex-valued symbols by performing DFT oninput symbols. For example, when Ntx symbols are input (where Ntx is anatural number), a DFT size is Ntx. The DFT unit 51 may be called atransform precoder. The subcarrier mapper 52 maps the complex-valuedsymbols to the respective subcarriers of the frequency domain. Thecomplex-valued symbols may be mapped to resource elements correspondingto a resource block allocated for data transmission. The subcarriermapper 52 may be called a resource element mapper. The IFFT unit 53outputs a baseband signal for data (that is, a time domain signal) byperforming IFFT on the input symbols. The CP insertion unit 54 copiessome of the rear part of the baseband signal for data and inserts thecopied parts into the former part of the baseband signal for data.Orthogonality may be maintained even in a multi-path channel becauseinter-symbol interference (ISI) and inter-carrier interference (ICI) areprevented through CP insertion.

FIG. 7 shows an example of a scheme in which the subcarrier mapper mapsthe complex-valued symbols to the respective subcarriers of thefrequency domain. Referring to FIG. 7( a), the subcarrier mapper mapsthe complex-valued symbols, outputted from the DFT unit, to subcarrierscontiguous to each other in the frequency domain. ‘0’ is inserted intosubcarriers to which the complex-valued symbols are not mapped. This iscalled localized mapping. In a 3GPP LTE system, a localized mappingscheme is used. Referring to FIG. 7( b), the subcarrier mapper insertsan (L−1) number of ‘0’ every two contiguous complex-valued symbols whichare outputted from the DFT unit (L is a natural number). That is, thecomplex-valued symbols outputted from the DFT unit are mapped tosubcarriers distributed at equal intervals in the frequency domain. Thisis called distributed mapping. If the subcarrier mapper uses thelocalized mapping scheme as in FIG. 7( a) or the distributed mappingscheme as in FIG. 7( b), a single carrier characteristic is maintained.

FIG. 8 shows an example of the structure of a reference signaltransmitter for demodulation.

Referring to FIG. 8 the reference signal transmitter 60 includes asubcarrier mapper 61, an IFFT unit 62, and a CP insertion unit 63.Unlike the transmitter 50 of FIG. 6, in the reference signal transmitter60, a reference signal is directly generated in the frequency domainwithout passing through the DFT unit 51 and then mapped to subcarriersthrough the subcarrier mapper 61. H ere, the subcarrier mapper may mapthe reference signal to the subcarriers using the localized mappingscheme of FIG. 7( a).

FIG. 9 shows examples of a subframe through which a reference signal istransmitted. The structure of the subframe in FIG. 9( a) shows a case ofa normal CP. The subframe includes a first slot and a second slot. Eachof the first slot and the second slot includes 7 OFDM symbols. The 14OFDM symbols within the subframe are assigned respective symbol indices0 to 13. A reference signal may be transmitted through the OFDM symbolshaving the symbol indices 3 and 10. Data may be transmitted through theremaining OFDM symbols other than the OFDM symbols through which thereference signal is transmitted. The structure of a subframe in FIG. 9(b) shows a case of an extended CP. The subframe includes a first slotand a second slot. Each of the first slot and the second slot includes 6OFDM symbols. The 12 OFDM symbols within the subframe are assignedsymbol indices 0 to 11. A reference signal is transmitted through theOFDM symbols having the symbol indices 2 and 8. Data is transmittedthrough the remaining OFDM symbols other than the OFDM symbols throughwhich the reference signal is transmitted.

Although not shown in FIG. 9, a sounding reference signal (SRS) may betransmitted through the OFDM symbols within the subframe. The SRS is areference signal for UL scheduling which is transmitted from a UE to aBS. The BS estimates a UL channel through the received SRS and uses theestimated UL channel in UL scheduling.

A clustered DFT-s OFDM transmission scheme is a modification of theexisting SC-FDMA transmission scheme and is a method of dividing datasymbols, subjected to a precoder, into a plurality of subblocks,separating the subblocks, and mapping the subblocks in the frequencydomain.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDMtransmission scheme. Referring to FIG. 10, the transmitter 70 includes aDFT unit 71, a subcarrier mapper 72, an IFFT unit 73, and a CP insertionunit 74. The transmitter 70 may further include a scramble unit (notshown), a modulation mapper (not shown), a layer mapper (not shown), anda layer permutator (not shown), which may be placed in front of the DFTunit 71.

Complex-valued symbols outputted from the DFT unit 71 are divided into Nsubblocks (N is a natural number). The N subblocks may be represented bya subblock #1, a subblock #2, . . . , a subblock #N. The subcarriermapper 72 distributes the N subblocks in the frequency domain and mapsthe N subblocks to subcarriers. The NULL may be inserted every twocontiguous subblocks. The complex-valued symbols within one subblock maybe mapped to subcarriers contiguous to each other in the frequencydomain. That is, the localized mapping scheme may be used within onesubblock.

The transmitter 70 of FIG. 10 may be used both in a single carriertransmitter or a multi-carrier transmitter. If the transmitter 70 isused in the single carrier transmitter, all the N subblocks correspondto one carrier. If the transmitter 70 is used in the multi-carriertransmitter, each of the N subblocks may correspond to one carrier.Alternatively, even if the transmitter 70 is used in the multi-carriertransmitter, a plurality of subblocks of the N subblocks may correspondto one carrier. Meanwhile, in the transmitter 70 of FIG. 10, a timedomain signal is generated through one IFFT unit 73. Accordingly, inorder for the transmitter 70 of FIG. 10 to be used in a multi-carriertransmitter, subcarrier intervals between contiguous carriers in acontiguous carrier allocation situation must be aligned.

FIG. 11 shows another example of a transmitter using the clustered DFT-sOFDM transmission scheme. Referring to FIG. 11, the transmitter 80includes a DFT unit 81, a subcarrier mapper 82, a plurality of IFFTunits 83-1, 83-2, . . . , 83-N (N is a natural number), and a CPinsertion unit 84. The transmitter 80 may further include a scrambleunit (not shown), a modulation mapper (not shown), a layer mapper (notshown), and a layer permutator (not shown), which may be placed in frontof the DFT unit 71.

IFFT is individually performed on each of N subblocks. An nth IFFT unit83-n outputs an nth baseband signal (n=1, 2, . . . , N) by performingIFFT on a subblock #n. The nth baseband signal is multiplied by an nthcarrier signal to produce an nth radio signal. After the N radio signalsgenerated from the N subblocks are added, a CP is inserted by the CPinsertion unit 84. The transmitter 80 of FIG. 11 may be used in adiscontinuous carrier allocation situation where carriers allocated tothe transmitter are not contiguous to each other.

FIG. 12 is yet another example of a transmitter using the clusteredDFT-s OFDM transmission scheme. FIG. 12 is a chunk-specific DFT-s OFDMsystem performing DFT precoding on a chunk basis. This may be called NxSC-FDMA. Referring to FIG. 12, the transmitter 90 includes a code blockdivision unit 91, a chunk division unit 92, a plurality of channelcoding units 93-1, . . . , 93-N, a plurality of modulators 94-1, . . . ,94-N, a plurality of DFT units 95-1, 95-N, a plurality of subcarriermappers 96-1, . . . , 96-N, a plurality of IFFT units 97-1, . . . ,97-N, and a CP insertion unit 98. Here, N may be the number of multiplecarriers used by a multi-carrier transmitter. Each of the channel codingunits 93-1, . . . , 93-N may include a scramble unit (not shown). Themodulators 94-1, . . . , 94-N may also be called modulation mappers. Thetransmitter 90 may further include a layer mapper (not shown) and alayer permutator (not shown) which may be placed in front of the DFTunits 95-1, . . . , 95-N.

The code block division unit 91 divides a transmission block into aplurality of code blocks. The chunk division unit 92 divides the codeblocks into a plurality of chunks. Here, the code block may be datatransmitted by a multi-carrier transmitter, and the chunk may be a datapiece transmitted through one of multiple carriers. The transmitter 90performs DFT on a chunk basis. The transmitter 90 may be used in adiscontinuous carrier allocation situation or a contiguous carrierallocation situation.

A UL reference signal is described below.

A reference signal is generally transmitted as a sequence. A referencesignal sequence is not particularly limited and a certain sequence maybe used as the reference signal sequence. As the reference signalsequence, a sequence generated through a computer based on phase shiftkeying (PSK) (i.e., a PSK-based computer generated sequence) may beused. The PSK may include, for example, binary phase shift keying(BPSK), quadrature phase shift keying (QPSK), and the like. Or, as thereference signal sequence, a constant amplitude zero auto-correlation(CAZAC) may be used. The CAZAC sequence may include, for example, aZadoff-Chu (ZC)-based sequence, a ZC sequence with cyclic extension, aZC sequence with truncation, and the like. Also, as the reference signalsequence, a pseudo-random (PN) sequence may be used. The PN sequence mayinclude, for example, an m-sequence, a sequence generated through acomputer, a gold sequence, a Kasami sequence, and the like. Also, acyclically shifted sequence may be used as the reference signalsequence.

A UL reference signal may be divided into a demodulation referencesignal (DMRS) and a sounding reference signal (SRS). The DMRS is areference signal used in channel estimation for the demodulation of areceived signal. The DMRS may be associated with the transmission of aPUSCH or PUCCH. The SRS is a reference signal transmitted from a UE to aBS for UL scheduling. The BS estimates an UL channel through thereceived SRS and uses the estimated UL channel in UL scheduling. The SRSis not associated with the transmission of a PUSCH or PUCCH. The samekind of a basic sequence may be used for the DMRS and the SRS.Meanwhile, in UL multi-antenna transmission, precoding applied to theDMRS may be the same as precoding applied to a PUSCH. Cyclic shiftseparation is a primary scheme for multiplexing the DMRS. In an LTE-Asystem, the SRS may not be precoded and may be an antenna-specificreference signal.

A reference signal sequence r_(u,v) ^((α))(n) may be defined based on abasic sequence b_(u,v)(n) and a cyclic shift α according to Equation 2.

r _(u,v) ^((α))(n)=e ^(jαn) b _(u,v)(n), 0≦n<M _(sc) ^(RS)  [Equation 2]

In Equation 2, M_(sc) ^(RS)(1≦m≦N_(RB) ^(max,UL)) is the length of thereference signal sequence and M_(sc) ^(RS)=m*Ns_(c) ^(RB). N_(sc) ^(RB)is the size of a resource block indicated by the number of subcarriersin the frequency domain. N_(RB) ^(max,UL) indicates a maximum value of aUL bandwidth indicated by a multiple of N_(sc) ^(RB). A plurality ofreference signal sequences may be defined by differently applying acyclic shift value α from one basic sequence.

A basic sequence b_(u,v)(n) is divided into a plurality of groups. Here,u □{0, 1, . . . , 29} indicates a group index, and v indicates a basicsequence index within the group. The basic sequence depends on thelength M_(sc) ^(RS) of the basic sequence. Each group includes a basicsequence (v=0) having a length of M_(sc) ^(RS) for m (1≦m≦5) andincludes 2 basic sequences (v=0,1) having a length of M_(sc) ^(RS) for m(6≦m≦n_(RB) ^(max,UL)). The sequence group index u and the basicsequence index v within a group may vary according to time as in grouphopping or sequence hopping.

Furthermore, if the length of the reference signal sequence is 3N_(sc)^(RB) or higher, the basic sequence may be defined by Equation 3.

b _(u,v)(n)=x _(q)(n mod N _(ZC) ^(RS)), 0≦n<M _(sc) ^(RS)  [Equation 3]

In Equation 3, q indicates a root index of a Zadoff-Chu (ZC) sequence.N_(ZC) ^(RS) is the length of the ZC sequence and may be a maximum primenumber smaller than M_(sc) ^(RS). The ZC sequence having the root indexq may be defined by Equation 4.

$\begin{matrix}{{{x_{q}(m)} = ^{{- j}\frac{\pi \; {{qm}{({m + 1})}}}{N_{ZC}^{RS}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

q may be given by Equation 5.

q==└ q+½┘+ν·(−1)^(└)2 q┘

q=N _(ZC) ^(RS)·(u+1)/31  [Equation 5]

If the length of the reference signal sequence is 3N_(sc) ^(RB) or less,the basic sequence may be defined by Equation 6.

b _(u,v) =e ^(jφ(n)π/4), 0≦n<M _(sc) ^(RS)−1  [Equation 6]

Table 1 is an example where φ(n) is defined when M_(sc) ^(RS)=N_(sc)^(RB).

TABLE 1 φ(0), . . . , φ(11) 0 −1 1 3 −3 3 3 1 1 3 1 −3 3 1 1 1 3 3 3 −11 −3 −3 1 −3 3 2 1 1 −3 −3 −3 −1 −3 −3 1 −3 1 −1 3 −1 1 1 1 1 −1 −3 −3 1−3 3 −1 4 −1 3 1 −1 1 −1 −3 −1 1 −1 1 3 5 1 −3 3 −1 −1 1 1 −1 −1 3 −3 16 −1 3 −3 −3 −3 3 1 −1 3 3 −3 1 7 −3 −1 −1 −1 1 −3 3 −1 1 −3 3 1 8 1 −33 1 −1 −1 −1 1 1 3 −1 1 9 1 −3 −1 3 3 −1 −3 1 1 1 1 1 10 −1 3 −1 1 1 −3−3 −1 −3 −3 3 −1 11 3 1 −1 −1 3 3 −3 1 3 1 3 3 12 1 −3 1 1 −3 1 1 1 −3−3 −3 1 13 3 3 −3 3 −3 1 1 3 −1 −3 3 3 14 −3 1 −1 −3 −1 3 1 3 3 3 −1 115 3 −1 1 −3 −1 −1 1 1 3 1 −1 −3 16 1 3 1 −1 1 3 3 3 −1 −1 3 −1 17 −3 11 3 −3 3 −3 −3 3 1 3 −1 18 −3 3 1 1 −3 1 −3 −3 −1 −1 1 −3 19 −1 3 1 3 1−1 −1 3 −3 −1 −3 −1 20 −1 −3 1 1 1 1 3 1 −1 1 −3 −1 21 −1 3 −1 1 −3 −3−3 −3 −3 1 −1 −3 22 1 1 −3 −3 −3 −3 −1 3 −3 1 −3 3 23 1 1 −1 −3 −1 −3 1−1 1 3 −1 1 24 1 1 3 1 3 3 −1 1 −1 −3 −3 1 25 1 −3 3 3 1 3 3 1 −3 −1 −13 26 1 3 −3 −3 3 −3 1 −1 −1 3 −1 −3 27 −3 −1 −3 −1 −3 3 1 −1 1 3 −3 −328 −1 3 −3 3 −1 3 3 −3 3 3 −1 −1 29 3 −3 −3 −1 −1 −3 −1 3 −3 3 1 −1

Table 2 is an example where φ(n) is defined when M_(sc) ^(RS)=2*N_(sc)^(RB).

TABLE 2 φ(0), . . . , φ(23) 0 −1 3 1 −3 3 −1 1 3 −3 3 1 3 −3 3 1 1 −1 13 −3 3 −3 −1 −3 1 −3 3 −3 −3 −3 1 −3 −3 3 −1 1 1 1 3 1 −1 3 −3 −3 1 3 11 −3 2 3 −1 3 3 1 1 −3 3 3 3 3 1 −1 3 −1 1 1 −1 −3 −1 −1 1 3 3 3 −1 −3 11 3 −3 1 1 −3 −1 −1 1 3 1 3 1 −1 3 1 1 −3 −1 −3 −1 4 −1 −1 −1 −3 −3 −1 11 3 3 −1 3 −1 1 −1 −3 1 −1 −3 −3 1 −3 −1 −1 5 −3 1 1 3 −1 1 3 1 −3 1 −31 1 −1 −1 3 −1 −3 3 −3 −3 −3 1 1 6 1 1 −1 −1 3 −3 −3 3 −3 1 −1 −1 1 −1 11 −1 −3 −1 1 −1 3 −1 −3 7 −3 3 3 −1 −1 −3 −1 3 1 3 1 3 1 1 −1 3 1 −1 1 3−3 −1 −1 1 8 −3 1 3 −3 1 −1 −3 3 −3 3 −1 −1 −1 −1 1 −3 −3 −3 1 −3 −3 −31 −3 9 1 1 −3 3 3 −1 −3 −1 3 −3 3 3 3 −1 1 1 −3 1 −1 1 1 −3 1 1 10 −1 1−3 −3 3 −1 3 −1 −1 −3 −3 −3 −1 −3 −3 1 −1 1 3 3 −1 1 −1 3 11 1 3 3 −3 −31 3 1 −1 −3 −3 −3 3 3 −3 3 3 −1 −3 3 −1 1 −3 1 12 1 3 3 1 1 1 −1 −1 1 −33 −1 1 1 −3 3 3 −1 −3 3 −3 −1 −3 −1 13 3 −1 −1 −1 −1 −3 −1 3 3 1 −1 1 33 3 −1 1 1 −3 1 3 −1 −3 3 14 −3 −3 3 1 3 1 −3 3 1 3 1 1 3 3 −1 −1 −3 1−3 −1 3 1 1 3 15 −1 −1 1 −3 1 3 −3 1 −1 −3 −1 3 1 3 1 −1 −3 −3 −1 −1 −3−3 −3 −1 16 −1 −3 3 −1 −1 −1 −1 1 1 −3 3 1 3 3 1 −1 1 −3 1 −3 1 1 −3 −117 1 3 −1 3 3 −1 −3 1 −1 −3 3 3 3 −1 1 1 3 −1 −3 −1 3 −1 −1 −1 18 1 1 11 1 −1 3 −1 −3 1 1 3 −3 1 −3 −1 1 1 −3 −3 3 1 1 −3 19 1 3 3 1 −1 −3 3 −13 3 3 −3 1 −1 1 −1 −3 −1 1 3 −1 3 −3 −3 20 −1 −3 3 −3 −3 −3 −1 −1 −3 −1−3 3 1 3 −3 −1 3 −1 1 −1 3 −3 1 −1 21 −3 −3 1 1 −1 1 −1 1 −1 3 1 −3 −1 1−1 1 −1 −1 3 3 −3 −1 1 −3 22 −3 −1 −3 3 1 −1 −3 −1 −3 −3 3 −3 3 −3 −1 13 1 −3 1 3 3 −1 −3 23 −1 −1 −1 −1 3 3 3 1 3 3 −3 1 3 −1 3 −1 3 3 −3 3 1−1 3 3 24 1 −1 3 3 −1 −3 3 −3 −1 −1 3 −1 3 −1 −1 1 1 1 1 −1 −1 −3 −1 325 1 −1 1 −1 3 −1 3 1 1 −1 −1 −3 1 1 −3 1 3 −3 1 1 −3 −3 −1 −1 26 −3 −11 3 1 1 −3 −1 −1 −3 3 −3 3 1 −3 3 −3 1 −1 1 −3 1 1 1 27 −1 −3 3 3 1 1 3−1 −3 −1 −1 −1 3 1 −3 −3 −1 3 −3 −1 −3 −1 −3 −1 28 −1 −3 −1 −1 1 −3 −1−1 1 −1 −3 1 1 −3 1 −3 −3 3 1 1 −1 3 −1 −1 29 1 1 −1 −1 −3 −1 3 −1 3 −11 3 1 −1 3 1 3 −3 −3 1 −1 −1 1 3

Hopping of a reference signal may be applied as follows.

The sequence group index u of a slot index ns may be defined based on agroup hopping pattern f_(gh)(n_(s)) and a sequence shift pattern f_(ss)according to Equation 7.

u=(f _(gh)(n _(s))+f _(ss))mod 30  [Equation 7]

17 different group hopping patterns and 30 different sequence shiftpatterns may exist. Whether to apply group hopping may be indicated by ahigher layer.

A PUCCH and a PUSCH may have the same group hopping pattern. A grouphopping pattern f_(gh)(n_(s)) may be defined by Equation 8.

$\begin{matrix}{{f_{gh}\left( n_{s} \right)} = \left\{ \begin{matrix}0 & {{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {disabled}} \\{\left( {\sum\limits_{i = 0}^{7}{{c\left( {{8n_{s}} + i} \right)} \cdot 2^{i}}} \right){mod}\mspace{14mu} 30} & {{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {enabled}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In Equation 8, c(i) is a pseudo random sequence that is a PN sequenceand may be defined by a Gold sequence of a length-31. Equation 9 showsan example of a gold sequence c(n).

c(n)=(x ₁(n+N _(c))+x ₂(n+N _(c)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₁(n+1)+x ₁(n))mod 2  [Equation 9]

Here, Nc=1600, x₁(i) is a first m-sequence, and x₂(i) is a secondm-sequence. For example, the first m-sequence or the second m-sequencemay be initialized according to a cell identifier (ID) for every OFDMsymbol, a slot number within one radio frame, an OFDM symbol indexwithin a slot, and the type of a CP. A pseudo random sequence generatormay be initialized to

$c_{init} = \left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor$

in the first of each radio frame.

A PUCCH and a PUSCH may have the same sequence shift pattern. Thesequence shift pattern of the PUCCH may be f_(ss) ^(PUCCH)=N_(ID)^(cell) mod 30. The sequence shift pattern of the PUSCH may be f_(ss)^(PUCCH)=(f_(ss) ^(PUCCH)+Δ_(ss)) mod 30 and Δ_(ss)□{0, 1, . . . , 29}may be configured by a higher layer.

Sequence hopping may be applied to only a reference signal sequencehaving a length longer than 6N_(sc) ^(RB). Here, a basic sequence indexv within a basic sequence group of a slot index ns may be defined byEquation 10.

$\begin{matrix}{v = \left\{ \begin{matrix}{c\left( n_{s} \right)} & {\mspace{14mu} \begin{matrix}{{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {disabled}\mspace{14mu} {and}} \\{{sequence}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {enabled}}\end{matrix}} \\0 & {otherwise}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

c(i) may be represented by an example of Equation 9. Whether to applysequence hopping may be indicated by a higher layer. A pseudo randomsequence generator may be initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$

in the first of each radio frame.

A DMRS sequence for a PUSCH may be defined by Equation 11.

r ^(PUSCH)(m·M _(sc) ^(RS) +n)=r _(u,v) ^((α))(n)  [Equation 11]

In Equation 11, m=0, 1, . . . and n=0, . . . , M_(sc) ^(RS)−1. M_(sc)^(RS)=M_(sc) ^(PUSCH).

α=2πn_(cs)/12, that is, a cyclic shift value is given within a slot, andn_(cs) may be defined by Equation 12.

n _(cs)=(n _(DMRS) ⁽¹⁾ +n _(DMRS) ⁽²⁾ +n _(PRS)(n _(s)))mod 12

In Equation 12, n_(DMRS) ⁽¹⁾ is indicated by a parameter transmitted bya higher layer, and Table 3 shows an example of a correspondingrelationship between the parameter and n_(DMRS) ⁽¹⁾.

TABLE 3 Parameter n_(DMRS) ⁽¹⁾ 0 0 1 2 2 3 3 4 4 6 5 8 6 9 7 10

Back in Equation 12, n_(DMRS) ⁽²⁾ may be defined by a cyclic shift fieldwithin a DCI format 0 for a transmission block corresponding to PUSCHtransmission. The DCI format is transmitted in a PDCCH. The cyclic shiftfield may have a length of 3 bits.

Table 4 shows an example of a corresponding relationship between thecyclic shift field and n_(DMRS) ⁽²⁾.

TABLE 4 Cyclic shift field in DCI format 0 n_(DMRS) ⁽²⁾ 000 0 001 6 0103 011 4 100 2 101 8 110 10 111 9

If a PDCCH including the DCI format 0 is not transmitted in the sametransmission block, if the first PUSCH is semi-persistently scheduled inthe same transmission block, or if the first PUSCH is scheduled by arandom access response grant in the same transmission block, n_(DMRS)⁽²⁾ may be 0.

n_(PRS)(n_(s)) may be defined by Equation 13.

n _(PRS)(n _(s))=Σ_(i=0) ⁷ c(8N _(symb) ^(UL) ·n _(s)+i)·2^(i)  [Equation 13]

c(i) may be represented by the example of Equation 9 and may be appliedin a cell-specific way of c(i). A pseudo random sequence generator maybe initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$

in the first of each radio frame.

A DMRS sequence r^(PUSCH) is multiplied by an amplitude scaling factorβ_(PUSCH) and mapped to a physical transmission block, used in relevantPUSCH transmission, from r^(PUSCH)(0) in a sequence starting. The DMRSsequence is mapped to a fourth OFDM symbol (OFDM symbol index 3) in caseof a normal CP within one slot and mapped to a third OFDM symbol (OFDMsymbol index 2) within one slot in case of an extended CP.

An SRS sequence r_(SRS)(n)=r_(u,v) ^((α))(n) is defined. u indicates aPUCCH sequence group index, and v indicates a basic sequence index. Thecyclic shift value α is defined by Equation 14.

$\begin{matrix}{\alpha = {2\; \pi \frac{n_{SRS}^{cs}}{8}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

n_(SRS) ^(cs) is a value configured by a higher layer in related to eachUE and may be any one of integers from 0 to 7.

Meanwhile, an orthogonal code cover (OCC) can be applied to a referencesignal sequence. The OCC implies a code having orthogonality andapplicable to a sequence. Although different sequences can be used todistinguish a plurality of channels in general, the OCC can be used todistinguish the plurality of channels.

The OCC can be used for the following purposes.

1) The OCC can be applied to increase an amount of radio resourcesallocated to an uplink reference signal.

For example, when a cyclic shift value of a reference signal transmittedin 1^(st) and 2^(nd) slots is assigned ‘a’, a minus sign (−) can beassigned to the reference signal transmitted in the 2^(nd) slot. Thatis, a 1^(st) user can transmit a reference signal having a cyclic shiftvalue ‘a’ and assigned a plus sign (+) in the 2^(nd) slot, and a 2^(nd)user can transmit a reference signal having the cyclic shift value ‘a’and assigned the minus sign (−) in the 2^(nd) slot. A BS can estimate achannel of the 1^(st) user by adding the reference signal transmitted inthe 1^(st) slot and the reference signal transmitted in the 2^(nd) slot.Further, the BS can estimate a channel of the 2^(nd) user by subtractingthe reference signal transmitted in the 2^(nd) slot from the referencesignal transmitted in the 1^(st) slot. That is, by applying the OCC, theBS can distinguish the reference signal transmitted by the 1^(st) userand the reference signal transmitted by the 2^(nd) user. Accordingly,since at least two users use the same reference signal while usingdifferent OCCs, an amount of available radio resources can be increasedby two-fold.

When transmitting an uplink reference signal by applying the OCC, afield for indicating the applied OCC can be allocated in a downlinkcontrol signal. For example, when it is assumed that an OCC indicatorfield is allocated with a 1-bit length in the downlink control signal,the OCC indicator can be expressed by Table 5.

TABLE 5 1^(st) slot 2^(nd) slot 0 1 1 1 1 −1

Referring to Table 5, when a value of the OCC indicator is 0, a plussign (+) is applied to a reference signal transmitted in the 2^(nd)slot, and when the value of the OCC indicator is 1, a minus sign (−) isapplied to a reference signal transmitted in the 2^(nd) slot.

2) The OCC can be applied to increase an interval of cyclic shift valuesallocated to multiple antennas or multiple layers of a single user.Although the cyclic shift values allocated to the multiple layers aredescribed hereinafter, the present invention can also apply to thecyclic shift values allocated to the multiple antennas.

An uplink reference signal distinguishes a channel on the basis of acyclic shift value. In a multi-antenna system, in order to distinguish aplurality of layers, different cyclic shift values can be allocated toreference signals of the respective layers. The cyclic shift value to beallocated must be increased in proportion to the number of layers, andthus an interval between cyclic shift values is decreased. Accordingly,it becomes difficult to distinguish the plurality of channels, therebydecreasing channel estimation capability. To overcome this problem, anOCC can be applied to each layer. For example, assume that cyclic shiftoffsets of reference signals for the layers are respectively allocatedto 0, 6, 3, and 9 for four antennas. An interval of the cyclic shiftvalues between the reference signals for the respective layers is 3. Inthis case, the interval of the cyclic shift values between the referencesignals of the respective layers can be increased to 6 by applying anOCC with a minus sign (−) to 3^(rd) and 4^(th) layers. That is, whenreference signal sequences with a length N and applied to a 1^(St) slotof 1^(st) to 4^(th) layers are respectively denoted by (S01, . . . ,S0N), (S61, . . . , S6N), (S31, . . . , S3N), and (S91, . . . , S9N),reference signal sequences applied to a 2^(nd) slot of the 1^(st) to4^(th) layers are respectively (S01, . . . , S0N), (S61, . . . , S6N),(−S31, . . . , −S3N), and (−S91, . . . , −S9N). When the referencesignal sequences of the two slots are added, only reference signals ofthe 1^(st) and 2^(nd) layers remain, and thus an interval of cyclicshift values is 6. Likewise, when the reference signal sequences of thetwo slots are subtracted, only reference signals of the 3^(rd) and4^(th) layers remain, and thus the interval of cyclic shift values isalso 6. Accordingly, channel estimation capability can be increased.

Likewise, assume that cyclic shift offsets of reference signals for thelayers are respectively allocated to 0, 6, and 3 for three layers. Aninterval of the cyclic shift values between the reference signals forthe respective layers is 3. In this case, the interval of the cyclicshift values between the reference signals of the respective layers canbe increased to 6 by applying an OCC with the minus sign (−) to a 3^(rd)layer. That is, when reference signal sequences with a length N andapplied to a 1^(st) slot of 1^(st) to 3^(rd) layers are respectivelydenoted by (S01, . . . , S0N), (S61, . . . , S6N), and (S31, . . . ,S3N), reference signal sequences applied to a 2^(nd) slot of the 1^(st)to 3^(rd) layers are respectively (S01, . . . , S0N), (S61, . . . ,S6N), (−S31, . . . , −S3N). When the reference signal sequences of thetwo slots are added, only reference signals of the 1^(st) and 2^(nd)layers remain, and thus an interval of cyclic shift values is 6.Likewise, when the reference signal sequences of the two slots aresubtracted, only a reference signal of the 3^(rd) layer remains.Accordingly, channel estimation capability can be increased.

3) The OCC can be applied to increase an interval of cyclic shift valuesallocated to a single user.

In a multi user-MIMO (MU-MIMO) system having multiple antennas andincluding a plurality of users, the OCC can be applied to a cyclic shiftvalue. For example, from the perspective of the single user whichperforms MIMO transmission, in order to distinguish a plurality ofantennas or a plurality of layers, a cyclic shift value having a greatinterval can be allocated between the respective antennas or therespective layers, whereas from the perspective of multiple users, acyclic shift interval between the respective users can be decreased. Toovercome this problem, the OCC can be applied. When the OCC is applied,the same cyclic shift value can be applied between multiple usersaccording to an OCC type.

Table 6 shows an example of applying the OCC when there are fourantennas or four layers.

TABLE 6 Types Type 1 Type 2 Type 3 Type 4 Layer/Antenna A B A B A B A B1 (1, 1) (1, −1) (1, 1) (1, −1) (1, 1)  (1, −1) (1, 1)  (1, −1) 2 (1, 1)(1, −1) (1, 1) (1, −1) (1, −1) (1, 1)  (1, −1) (1, 1)  3 (1, 1) (1, −1) (1, −1) (1, 1)  (1, 1)  (1, −1) (1, −1) (1, 1)  4 (1, 1) (1, −1)  (1,−1) (1, 1)  (1, −1) (1, 1)  (1, −1) (1, −1)

In Table 6, (a,b) denotes an OCC applied to (1^(st) slot, 2^(nd) slot)or (2^(nd) slot, 1^(st) slot). A 1-bit OCC type field for indicating atype of applying the OCC can be added to a downlink control signal forindicating a cyclic shift value.

Table 7 shows an example of an OCC type field.

TABLE 7 Codeword of OCC Type 0 A (/B) 1 B (/A)

In Table 7, if a value of the OCC type field is 0, the type A- (ortype-B) OCC of Table 6 can be applied, and if the value of the OCC typefield is 1, the type B-(or type-A) OCC of Table 6 can be applied.

Referring to the type 1-B of Table 6, the minus sign (−) is applied toall layers' or antennas' reference signals transmitted in any one slot.As such, when the OCC is applied, the OCC may be applied to some usersand the OCC may not be applied to the other users. The OCC can beutilized as a resource, or can be used to increase an interval of cyclicshift values between multiple users.

Referring to the type 2-A of Table 6, the minus sign (−) is applied tosome layers' or antennas' reference signals transmitted in any one slot.In the type 2-A, the minus sign (−) is applied to a reference signal ofa 3^(rd) layer (or antenna) or a 4^(th) layer (or antenna). The OCC canbe utilized as a resource, or can be used to increase an interval ofcyclic shift values between multiple users.

Table 8 shows an example of applying the type-2 OCC of Table 6 to twousers.

TABLE 8 1^(st) Slot 2^(nd) Slot UE 1 0 6 3 9 0 6 −3 −9 UE 2 4 10 4 10

The 1^(st) user transmits a reference signal with respect to fourlayers, and the 2^(nd) user transmits a reference signal with respect totwo layers. The type 2-A OCC of Table 6 is applied to both the 1^(st)and 2^(nd) users. Accordingly, the minus sign (−) is applied toreference signals of 3^(rd) and 4^(th) layers of the 1^(st) user, andthe minus sign (−) is not applied to reference signals of 1^(st) and2^(nd) layers of the 2^(nd) user.

Referring to the type 3-A of Table 6, the minus sign (−) is applied tosome layers' or antennas' reference signals transmitted in any one slot.In the type 3-A, the minus sign (−) is applied to a reference signal ofa 2^(nd) layer (or antenna) or a 4^(th) layer (or antenna). The OCC canbe utilized as a resource, or can be used to increase an interval ofcyclic shift values between multiple users.

Referring to the type 4-A of Table 6, the minus sign (−) is applied tosome layers' or antennas' reference signals transmitted in any one slot.In the type 4-A, the minus sign (−) is applied to a reference signal ofa 2^(nd) layer (or antenna) or a 3^(rd) layer (or antenna). The OCC canbe utilized as a resource, or can be used to increase an interval ofcyclic shift values between multiple users.

Hereinafter, the proposed reference signal transmission method will bedescribed. According to the proposed reference signal transmissionmethod, cyclic shift values for reference signals of a plurality oflayers or a plurality of antennas can be applied variously. Although acase where the cyclic shift values are allocated to the referencesignals of the plurality of layers is described hereinafter, the presentinvention is not limited thereto, and thus can also apply to a casewhere the cyclic shift values are allocated to the reference signals ofthe plurality of antennas.

First, cyclic shift values can constitute a set without considering theOCC, and thus can be allocated to reference signals of the plurality oflayers.

The cyclic shift values can be allocated by considering a singleuser-MIMO (SU-MIMO) system. Due to a characteristic of a referencesignal sequence used for uplink reference signal transmission of 3GPPLTE rel-8, a shift occurs in a time domain by a value corresponding toan allocated cyclic shift value. For example, if an FFT size is 512,when an interval of cyclic shift values is 1, the interval correspondsto 43 samples in a time domain. Meanwhile, a channel impulse responseexists in a CP period in general. After receiving the channel impulseresponse existing in the CP period, it can be replaced with afrequency-domain signal to obtain an estimated channel. In case ofmulti-antenna transmission, a signal received from each antenna has asimilar delay in general, and the channel impulse response may exist inthe CP period or may be slightly deviated from the CP period. Therefore,by allocating a cyclic shift value having an interval equal to orgreater than 1 or 2 in an SU-MIMO environment, a channel impulseresponse experienced by a signal transmitted from each antenna can beobtained sufficiently without interference between antennas.Accordingly, a minimum interval of cyclic shift values betweenrespective layers is preferably equal to or greater than 1 in theSU-MIMO.

A set of cyclic shift values can be configured variously. For example,the set of cyclic shift values may be {0,2,3,4,6,8,9,10} which is a setof 8 cyclic shift values defined in 3GPP LTE rel-8. In a normal CP orextended CP, a cyclic shift value can be selected from the set. Inaddition, a subset of the set can be used. For example, a cyclic shiftvalue can be selected from the subset consisting of {0,3,6,9}. When achannel has a long delay spread, a subset consisting of cyclic shiftvalues having a great interval of the cyclic shift values can be used.

For another example, the set of cyclic shift values may be {0, 1, 2,3,4,5,6,7,8,9,10,11} which is a set of 12 cyclic shift values. Inaddition, a subset of the set can be used.

For another example, the set of cyclic shift values may be{0,4,8,2,6,10,3,9} which is a set of 8 cyclic shift values. In thiscase, in a cyclic shift value defined in 3GPP LTE rel-8, the cyclicshift value is selected such that cyclic shift values have an intervalof 4, and if the cyclic shift value is greater than 12, the cyclic shiftvalue is selected by performing a modulo operation. If there is apre-selected value, a value closest to the selected value can beselected. If the set of cyclic shift values is a set of 12 cyclic shiftvalues, the set may be {0,4,8,1,5,9,2,6,10}.

The cyclic shift set determined by using the aforementioned method canbe indicated through a cyclic shift indicator allocated to a DCI formattransmitted through a PDCCH. The cyclic shift indicator may have alength of 3 bits. By using a cyclic shift value indicated by the cyclicshift indicator as a start point of a cyclic shift set, and cyclic shiftoffsets can be allocated by the number of layers. That is, on the basisof the start point of the cyclic shift set and the cyclic shift offsetsallocated to respective layers, cyclic shift values of the respectivelayers can be determined. An allocation order of the cyclic shiftoffsets may be sequential or may conform to a predetermined rule. Thepredetermined rule may be any sequence or may be an order based on anoffset. The start point of the cyclic shift set indicated by the cyclicshift indicator may be any one of cyclic shift values allocated to therespective layers, or may be any one of cyclic shift offsets allocatedto the respective layers. Alternatively, the start point may be the samevalue as n_(DMRS) ⁽²⁾. For example, if a cyclic shift set is{0,2,3,4,6,8,9,10}, a cyclic shift indicator is 0, and the number oflayers is 2, then starting from a cyclic shift value 0 in the cyclicshift set, cyclic shift values 0 and 2 can be selected as cyclic shiftvalues of uplink reference signal sequences. Alternatively, if a cyclicshift set is {0,2,3,4,6,8,9,10}, a cyclic shift indicator is 0, thenumber of layers is 3, and cyclic shift offset values allocated to1^(st) to 3^(rd) layers are respectively {0,6,3}, then cyclic shiftvalues allocated to the 1^(st) to 3^(rd) layers may be respectively{0,6,3}.

In addition to the cyclic shift indicator, a selection offset can beadditionally allocated in a DCI format. Starting from a cyclic shiftvalue indicated by the cyclic shift indicator, cyclic shift values ofreference signal sequences for a plurality layers can be allocated withan interval corresponding to a value indicated by the selection offset.The selection offset may have a length of 1 bit or two bits. If theselection offset has a length of 1, the selection offset may be any oneof {1,2}, {1,3}, and {1,4}. If the selection offset has a length of 2,the selection offset may be any one of {1,2,3,4}. For example, if acyclic shift indicator is 3 bits, a selection offset is 1 bit, a cyclicshift set consists of {0,2,3,4,6,8,9,10}, a cyclic shift indicator and aselection offset used by a 1^(st) user are respectively ‘000’ and ‘0’,and a cyclic shift indicator and a selection offset used by a 2^(nd)user are respectively ‘101’ and ‘1’, then cyclic shift values ofreference signals of respective layers of the 1^(st) user may be {0,2},and cyclic shift values of reference signals of respective layers of the2^(nd) user may be {8,10}.

Meanwhile, if the number of layers is 3, two cyclic shift indicators canbe allocated from the DCI format and thus can be used as cyclic shiftvalues of reference signals of two layers, and a cyclic shift value of areference signal of the remaining one layer can be allocated based onany one of the two cyclic shift indicators indicated by a PDCCH. In thiscase, the cyclic shift value of the reference signal of the remainingone layer can be implicitly determined based on a selection offsetwithout additional signaling. Alternatively, the cyclic shift value ofthe reference signal of the remaining layer can be allocated based onany one of the two cyclic shift indicators.

The above description is also applied to a case where the number oflayers is 4.

Two cyclic shift indicators can be allocated from the DCI format andthus can be used as cyclic shift values of reference signals of twolayers, and cyclic shift values of reference signals of the remainingtwo layers can be allocated based on the two cyclic shift indicators.For example, a cyclic shift value of a reference signal of a 3^(rd)layer can be based on a cyclic shift value of a reference signal of a1^(st) layer, and a cyclic shift value of a reference signal of a 4^(th)layer can be based on a cyclic shift value of a reference signal of a2^(nd) layer. The cyclic shift values of the reference signals of theremaining two layers can be implicitly determined based on a selectionoffset without additional signaling.

Although allocation of a cyclic shift value of an uplink DMRS has beendescribed above by considering a plurality of layers, the presentinvention is not limited thereto, and thus can also apply to an uplinksounding reference signal. In this case, the present invention can applyspecifically to an uplink sounding reference signal by varying a cyclicshift indicator allocated for the DMRS, and a cyclic shift set, etc. Inaddition, a signaling overhead can be prevented from occurring bydirectly applying the cyclic shift indicator for the DMRS or the cyclicshift value to the sounding reference signal.

Hereinafter, a method of allocating a cyclic shift value of a referencesignal sequence of each layer by combining a cyclic shift index forindicating a cyclic shift value and an OCC index for indicating an OCCwill be described. In this case, the cyclic shift value can bedetermined such that an interval of cyclic shift values of referencesignals of respective layers is maximized. Alternatively, the cyclicshift value of the reference signals of the respective layers can bedetermined by using a cyclic shift indicator without additionalsignaling of the OCC index. In the following description, the cyclicshift index and the OCC index are described by using a table. Inaddition, although it is assumed that the number of layers is 4, whenthe number of layers is less than or equal to 4, it is also possible touse only cyclic shift values for some layers among cyclic shift valuesproposed in the corresponding table.

First, cyclic shift values can be allocated such that an interval ofcyclic shift values of reference signals between 1^(st) and 2^(nd)layers and an interval of cyclic shift values of reference signalsbetween 3^(rd) and 4^(th) layers are maximized. According to the appliedOCC, only reference signals of the 1^(st) and 2^(nd) layers may remainand channel estimation may be performed in this state, and on the otherhand, only reference signals of the 3^(rd) and 4^(th) layers may remainand channel estimation may be performed in this state.

Table 9 shows an example in which a cyclic shift index and an OCC indexare mapped according to the proposed invention.

TABLE 9 Cyclic Shift Field Index i(cyclic shift) in DCI format 0n_(DMRS) ⁽²⁾ OCC index 0 000 0 0 1 001 6 0 2 010 3 1 3 011 4 1 4 100 2 05 101 8 0 6 110 10 1 7 111 9 1

According to Table 9, a cyclic shift index i and an OCC index aremapped. A cyclic shift field in DCI format 0 indicated by the cyclicshift index and n_(DMRS) ⁽²⁾ to be mapped to the cyclic shift field aremapped to the OCC index. That is, the same OCC index is always appliedto the value n_(DMRS) ⁽²⁾. For example, if n_(DMRS) ⁽²⁾=0, the OCC indexmay be always 0, and if n_(DMRS) ⁽²⁾=3, the OCC index may be always 1.In this case, when the OCC index is 0, it implies that an OCC applied to1^(st) and 2^(nd) slots are [1 1], and when the OCC is 1, it impliesthat an OCC applied to the 1^(st) and 2^(nd) slots are [1 −1].Alternatively, the opposite is also applicable.

Table 10 shows a cyclic shift value of a reference signal of each layerapplied according to Table 9.

TABLE 10 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 6 −3 −9 1 001 6 6 0 −9−3 2 010 3 −3 −9 6 0 3 011 4 −4 −10 8 2 4 100 2 2 8 4 10 5 101 8 8 2 −10−4 6 110 10 −10 −4 2 8 7 111 9 −9 −3 0 6

In Table 10, a minus sign (−) implies that an OCC index 1 is applied andthus the minus sign (−) is applied to a reference signal transmitted ina 2^(nd) slot. According to Table 10, cyclic shift values of referencesignals of 1^(st) and 2^(nd) layers maintain a maximum interval, andlikewise cyclic shift values of reference signals of 3^(rd) and 4^(th)layers also maintain a maximum interval. When the number of layers is 2or 3, only some of cyclic shift values of Table 10 can be used.

Alternatively, the cyclic shift values of the reference signals of therespective layers can be allocated such that interference is reduced tothe maximum extent possible in rank-2 transmission. Although an intervalof the cyclic shift values of the reference signals of the respectivelayers is not maximized in rank-4 transmission, according to the appliedOCC, only reference signals of the 1^(st) and 3^(rd) layers may remainand channel estimation may be performed in this state, and on the otherhand, only reference signals of the 2^(nd) and 4^(th) layers may remainand channel estimation may be performed in this state.

Table 11 shows an example in which a cyclic shift index and an OCC indexare mapped according to the proposed invention.

TABLE 11 Cyclic Shift Field Index i(cyclic shift) in DCI format 0n_(DMRS) ⁽²⁾ OCC index 0 000 0 0 1 001 6 1 2 010 3 0 3 011 4 1 4 100 2 05 101 8 1 6 110 10 0 7 111 9 1

Table 12 shows a cyclic shift value of a reference signal of each layerapplied according to Table 10.

TABLE 12 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 −6 3 −9 1 001 6 −6 0 −93 2 010 3 3 −9 −6 0 3 011 4 −4 10 −8 2 4 100 2 2 8 4 10 5 101 8 8 2 −10−4 6 110 10 −10 −4 2 8 7 111 9 −9 −3 0 6

When the number of layers is 2 or 3, only some of cyclic shift values ofTable 12 can be used.

According to the number of layers, it is also possible to allocatecyclic shift values based on different rules. For example, the cyclicshift values of Table 10 can be allocated in case of rank-2transmission, and the cyclic shift values of Table 12 can be allocatedin case of rank-4 transmission. Alternatively, the cyclic shift valuesof Table 12 can be allocated in case of rank-2 transmission, and thecyclic shift values of Table 10 can be allocated in case of rank-4transmission.

The cyclic shift value can be allocated by combining a cyclic shiftvalue and an OCC. When the number of layers is 1, different cyclic shiftvalues can be allocated according to a cyclic shift index. However, incase of a plurality of layers, the same cyclic shift value can beallocated even though cyclic shift indices are different. For example,any one of {0,6,3,4,2,8,10,9} can be used as a cyclic shift value of areference signal of one layer, and any one of{(0,6),(6,0),(3,9),(4,10),(2,8),(8,2),(10,4),(9,3)} can be used ascyclic shift values of reference signals of two layers. In this case,(0,6)−(6,0)/(3,9)−(9,3)/(4,10)−(10,4)/(2,8)−(8,2) have the same cyclicshift value even though cyclic shift indices are different. Accordingly,in this case, the OCC can be applied to maintain orthogonality. Forexample, the OCC can be applied such as (0,6),(−6,−0). In this case, ifa minus sign (−) is applied to reference signals of 1^(st) and 2^(nd)layers, a plus sign (+) can be applied to reference signals of 3^(rd)and 4^(th) layers.

Table 13 shows an example of a cyclic shift value of a reference signalof each layer according to the proposed invention.

TABLE 13 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 6 −3 −9 1 001 6 −6 −0 93 2 010 3 3 9 −6 −0 3 011 4 −4 −10 8 2 4 100 2 −2 −8 4 10 5 101 8 8 2−10 4 6 110 10 10 4 −2 −8 7 111 9 −9 −3 0 6

Table 14 shows another example of a cyclic shift value of a referencesignal of each layer according to the proposed invention.

TABLE 14 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 6 3 9 1 001 6 6 0 9 3 2010 3 3 9 6 0 3 011 4 4 10 7 1 4 100 2 2 8 5 11 5 101 8 8 1 11 4 6 11010 10 4 1 7 7 111 9 9 3 0 6

Table 15 shows an example of applying the OCC to the reference signal ofthe 3^(rd) and 4^(th) layers of Table 14.

TABLE 15 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 6 −3 −9 1 001 6 6 0 −9−3 2 010 3 3 9 −6 −0 3 011 4 4 10 −7 −1 4 100 2 2 8 −5 −11 5 101 8 8 1−11 −4 6 110 10 10 4 −1 −7 7 111 9 9 3 −0 −6

Table 16 shows an example of applying the OCC to the reference signal ofthe 1′ layer of Table 14

TABLE 16 Index Cyclic Shift Cyclic shift Cyclic shift Cyclic shiftCyclic shift i(cyclic Field in DCI value of RS for value of RS for valueof RS for value of RS for shift) format 0 [3] n_(DMRS) ⁽²⁾ rank-1 indexrank-2 index rank-3 index rank-4 index 0 000 0 0 6 −3 −9 1 001 6 −6 −0 93 2 010 3 3 9 −6 −0 3 011 4 −4 −10 7 1 4 100 2 −2 −8 5 11 5 101 8 8 1−11 −4 6 110 10 10 4 −1 −7 7 111 9 −9 −3 0 6

When the number of layers is less than or equal to 4, only cyclic shiftvalues of reference signals of some layers may be allocated among thecyclic shift values of Table 13 to Table 16.

FIG. 13 is a block diagram showing an embodiment of the proposedreference signal transmission method.

In step S100, a UE generates a plurality of reference signal sequencesin which different cyclic shift values are allocated respectively to aplurality of layers. In step S110, the UE generates an SC-FDMA symbol towhich the plurality of reference signal sequences are mapped. In stepS120, the UE transmits the SC-FDMA symbol through a plurality ofantennas. The cyclic shift values allocated to the respective layers canbe determined based on a 1^(st) cyclic shift value which is a cyclicshift value allocated to a 1^(st) layer among the plurality of layersand different cyclic shift offsets allocated to the respective layers.

FIG. 14 is a block diagram of a UE according to an embodiment of thepresent invention.

A UE 900 includes a reference signal generator 910, an SC-FDMA symbolgenerator 920, and a radio frequency (RF) unit 930. The reference signalgenerator 910 generates a plurality of reference signal sequences inwhich different cyclic shift values are allocated respectively to aplurality of layers. The SC-FDMA symbol generator 920 is connected tothe reference signal generator and generates an SC-FDMA symbol to whichthe plurality of reference signal sequences are mapped. The RF unit 930is connected to the SC-FDMA symbol generator and transmits the SC-FDMAsymbol to a BS through a plurality of antennas.

The exemplary embodiments of the present invention may be implemented byhardware, software, or a combination thereof. The hardware may beimplemented by an application specific integrated circuit (ASIC),digital signal processing (DSP), a programmable logic device (PLD), afield programmable gate array (FPGA), a processor, a controller, amicroprocessor, other electronic units, or a combination thereof, all ofwhich are designed so as to perform the above-mentioned functions. Thesoftware may be implemented by a module performing the above-mentionedfunctions. The software may be stored in a memory unit and may beexecuted by a processor. The memory unit or a processor may adoptvarious units well-known to those skilled in the art.

In the above-mentioned exemplary embodiments, the methods are describedbased on the series of steps or the flow charts shown by a block, butthe exemplary embodiments of the present invention are not limited tothe order of the steps and any steps may be performed in order differentfrom the above-mentioned steps or simultaneously. In addition, a personskilled in the art to which the present invention pertains mayunderstand that steps shown in the flow chart are not exclusive andthus, may include other steps or one or more step of the flow chart maybe deleted without affecting the scope of the present invention.

The above-mentioned embodiments include examples of various aspects.Although all possible combinations showing various aspects are notdescribed, it may be appreciated by those skilled in the art that othercombinations may be made. Therefore, the present invention should beconstrued as including all other substitutions, alterations andmodifications belonging to the following claims.

1. A method of transmitting a demodulation reference signal (DMRS) for aphysical uplink shared channel (PUSCH) in a multi-layers system, themethod comprising: generating a plurality of DMRS sequences associatedwith a plurality of layers respectively, wherein different cyclic shiftsare allocated to the plurality of DMRS sequences associated with theplurality of layers respectively; generating a single carrier-frequencydivision multiple access (SC-FDMA) symbol to which mapping the pluralityof DMRS sequences to a set of time domain and frequency domain resourceelements (REs) to form a plurality of DMRS; and transmitting theplurality of DMRS to a base station, wherein the corresponding cyclicshift allocated to the corresponding DMRS sequence are determined by afirst cyclic shift value which constitutes a cyclic shift allocated to afirst DMRS sequence associated with a first layer among the plurality oflayers and different cyclic shift offsets allocated to the each of theplurality of DMRS sequences.
 2. The method of claim 1, wherein the firstcyclic shift value and a second cyclic shift value which constitutes acyclic shift allocated to a second DMRS sequence associated with asecond layer among the plurality of layers have a maximum interval. 3.The method of claim 1, wherein the number of the plurality of layers is3.
 4. The method of claim 1, wherein a third cyclic shift offset whichis a cyclic shift offset allocated to a third DMRS sequence associatedwith a third layer among the plurality of layers is a median value of afirst cyclic shift offset which is the cyclic shift offset allocated tothe first DMRS sequence associated with the first layer and a secondcyclic shift offset which is the cyclic shift offset allocated to asecond DMRS sequence associated with a second layer.
 5. The method ofclaim 4, wherein the first cyclic shift offset, the second cyclic shiftoffset, and the third cyclic shift offset are respectively 0, 6, and 3.6. The method of claim 1, wherein if the number of the plurality oflayers is 4, a third cyclic shift value which constitutes a cyclic shiftvalue allocated to a third DMRS sequence associated with a third layeramong the plurality of layers and a fourth cyclic shift value whichconstitutes a cyclic shift value allocated a fourth DMRS sequenceassociated with to a fourth layer have a maximum interval.
 7. The methodof claim 1, wherein the cyclic shift values for plurality of DMRSsequences associated with the plurality of layers are indicated by acyclic shift field in a downlink control information (DCI) formatreceived through a physical downlink control channel (PDCCH).
 8. Themethod of claim 7, wherein the cyclic shift field has a length of 3bits.
 9. The method of claim 1, wherein the plurality of DMRS sequencesis transmitted in two slots constituting a subframe.
 10. The method ofclaim 9, wherein the plurality of DMRS sequences are transmitted in afourth single carrier frequency division multiple access (SC-FDMA)symbol of each slot in case of a normal cyclic prefix (CP), and whereinthe plurality of DMRS sequences are transmitted in a third SC-FDMAsymbol of each slot in case of an extended CP.
 11. The method of claim1, wherein an orthogonal covering code (OCC) is applied to the pluralityof DMRS sequences associated with the plurality of layers.
 12. Anapparatus for transmitting a demodulation reference signal (DMRS) for aphysical uplink shared channel (PUSCH), the apparatus comprising: aradio frequency (RF) unit configured for transmitting or receiving aradio signal; and a processor coupled to the RF unit, wherein theprocessor configured for: generating a plurality of DMRS sequencesassociated with a plurality of layers respectively, wherein differentcyclic shifts are allocated to the plurality of DMRS sequencesassociated with the plurality of layers respectively; mapping theplurality of DMRS sequences to a set of time domain and frequency domainresource elements (REs) to form a plurality of DMRS; and transmittingthe plurality of DMRS to a base station, wherein the correspondingcyclic shift allocated to the corresponding DMRS sequence are determinedby a first cyclic shift value which constitutes a cyclic shift valueallocated to a first DMRS sequence associated with a first layer amongthe plurality of layers and different cyclic shift offsets allocated tothe each of the plurality of DMRS sequences.
 13. The apparatus of claim12, wherein the first cyclic shift value and a second cyclic shift valuewhich constitutes a cyclic shift allocated to a second DMRS sequenceassociated with a second layer among the plurality of layers have amaximum interval.
 14. The apparatus of claim 12, wherein a third cyclicshift offset which is a cyclic shift offset allocated to a third DMRSsequence associated with a third layer among the plurality of layers isa median value of a first cyclic shift offset which is the cyclic shiftoffset allocated to the first DMRS sequence associated with the firstlayer and a second cyclic shift offset which is the cyclic shift offsetallocated to a second DMRS sequence associated with a second layer. 15.The apparatus of claim 14, wherein the first cyclic shift offset, thesecond cyclic shift offset, and the third cyclic shift offset arerespectively 0, 6, and 3.